To maintain a desirable automotive internal combustion engine air/fuel ratio, such as the stoichiometric ratio, the amount of fuel delivered to the engine is determined in response to an estimated or measured engine cylinder inlet air mass. Mass airflow sensors are available for measuring cylinder inlet air mass directly under steady state conditions characterized by substantially no intake manifold filling or depletion, but are not generally responsive enough to provide for accurate cylinder inlet air mass information under transient conditions characterized, for example, by significant time rate of change in engine intake manifold air pressure. Speed density approaches are sufficiently responsive to provide accurate cylinder inlet air mass information during even severe engine transient conditions, and therefore are known to be useful as a supplement to mass airflow sensor-based approaches during transient conditions. However, conventional speed density approaches suffer shortcomings in inlet air mass measurement accuracy under certain engine operating conditions. Inaccurate engine cylinder inlet air mass measurement can lead to deviations in engine air/fuel ratio away from a desired air/fuel ratio, such as the stoichiometric ratio, leading to increased engine emissions and reduced engine performance. It would be desirable to resolve the accuracy shortcomings in conventional speed density based engine air/fuel ratio control approaches.
The speed density approaches provide engine cylinder inlet air mass m as a function of engine intake manifold pressure MAP, for example using the ideal gas law, which may be expressed as EQU m=MAP*V*VE/(R*T)
in which V is cylinder volume, VE is volumetric efficiency, R is the ideal gas constant, and T is air temperature. While the ideal gas law includes an air temperature term, the volumetric efficiency VE term applied with the ideal gas law to determine engine inlet air mass is conventionally determined using static calibration parameters. While such engine parameters as engine valve timing and engine cylinder port geometry, on which VE depends, do not change substantially during engine operation, other parameters on which VE depends, such as engine cylinder inlet air temperature, can change significantly during engine operation, resulting in substantial open-loop engine air/fuel ratio error. For example, the air/fuel ratio becomes lean when intake air temperature increases above a calibration temperature and becomes rich when intake air temperature decreases below the calibration temperature. Intake air temperature depends not only on ambient air temperature, but on heat transfer between the engine and the intake air resulting in intake air temperature change prior to entry into and combustion in the engine cylinders. Conventional underhood styling and packaging trends have generally resulted in a significant increase in ambient air heating in the intake air path. Cylinder valves are responsible for a significant portion of the heating of the intake air mass. The heating results in gas expansion which reduces air density and in gas turbulence which disrupts the flow of inlet air through cylinder intake ports. Such effects can reduce volumetric efficiency significantly away from calibration values. The amount of intake air heating is proportional to the intake airflow rate and to the temperature difference between the intake air and the engine. It would be desirable to compensate for such effects that drive actual volumetric efficiency away from calibration values, to improve engine air/fuel ratio control accuracy.